1. Technical Field
This invention relates to a security element for electronic article surveillance in an electromagnetic surveillance system, comprising a soft magnetic, magnetostrictive strip and a premagnetization element fabricated from a semi-hard or hard magnetic material (for example, SEMIVAC by the firm Vacuumschmelze) that is associated with the strip, and to a method of manufacturing such a security element.
2. Background Art
From EP 0 093 281 B1, a security element and a surveillance apparatus for a corresponding security element attached to the article subject to surveillance are already known. The security element is comprised of an elongated strip of a preferably amorphous magnetic material of high permeability and magnetostriction. The strip is designed to be incited to mechanical vibrations at the frequency of the incident alternating magnetic field of the surveillance apparatus. The elastic postoscillatory response and the change in the magnetization of the strip coupled thereto on account of the magnetostrictive phenomenon induce a voltage change in the receiver device.
By suitable alloy compositions it is possible to vary the strip's magnetostrictive properties in wide ranges. A feature common to all these alloys is that a more or less distinct preferred direction transverse to the strip's longitudinal axis can be impressed upon them by an appropriate heat treatment.
This transversal anisotropy is necessary to incite mechanical vibrations in the material with fields along the longitudinal axis of the strip. Because the length variation is a square function of the cosine of the change in the magnetization, the maximum length variation and thus the maximum magneto-elastic effect are obtained when the direction of the magnetization is adjusted to an approximately 45.degree. angle to the strip's longitudinal axis. This is accomplished by suitably premagnetizing the strip in an external field of sufficient intensity. In the absence of demagnetizing effects, an amount 0.7 times the intensity of the anisotropic field of the induced anisotropy would be required. Generally, however, lower field strengths are sufficient.
The intensity of premagnetization thus determines both the amplitude of the oscillation and the natural resonant frequency of the strip. Proper adjustment of the premagnetization is the absolute prerequisite for the obtainment of an optimum resonant response of the strip at the emitted interrogation frequency of the transmitter device. The term optimum as used herein means that the characteristic signal has a sufficiently high oscillation amplitude, in addition to dying out optimally.
For technical implementation of the premagnetization, it is proposed in EP 0 093 281 B1 to produce a stray field of sufficient intensity along the strip by means of one or several magnetic elements which are magnetized and arranged in close proximity to the strip. Deactivation of the security element is then accomplished simply by demagnetizing the security element or magnetizing the premagnetization element in antiparallel areas, causing the stray field to disappear. However, this method is not absolutely reliable. As a result of the still relatively high permeability of the not premagnetized strip, an alarm may be produced although deactivation has taken place in surveillance apparatus operating on the basis of a harmonic principle (=detection of harmonic waves), in particular where the hysteresis curve of the strip proceeds along a non-linear course.
In addition, the inhomogeneities in both the strip and the premagnetization elements may cause relatively severe fluctuations in the magneto-elastic properties, particularly the resonant frequency. As a result of the spread of the magnetic parameters (resonant frequency, decay) thereby produced, the risk then exists that the security element fails to be reliably detected by the surveillance apparatus.
To reduce the material-induced spread of the resonance properties, it is proposed in EP 0 690 425 A1 to cut the strip to the appropriate length during manufacture. This is a relatively complicated approach involving high manufacturing cost.
EP 0 696 784 A1 proposes circumventing this problem. As in EP 0 093 281 B1, a premagnetization element is used whose stray field is dimensioned such as to enable the magneto-elastic strip to be magnetized to a higher degree. For one purpose, this makes it possible to deactivate the security element such as to prevent it from being detected also in other surveillance systems; for another purpose, the resonant frequency can be tuned to the respective frequency of the interrogation field within specified limits by selected magnetization of the premagnetization element, thereby enabling inhomogeneities in the materials employed to be compensated for.
The disadvantage of this method is that for the purpose of optimizing the strip's magneto-elastic response it is necessary to adjust the magnetization of the premagnetization element individually by suitable selection of an external field.